1. Field of the Invention
The present invention relates to a printing apparatus capable of forming images on print sheets transported on transportation routes, and particularly relates to a printing apparatus capable of handling paper size error.
2. Description of the Background Art
Conventionally, when a printing apparatus such as an ink jet printer transports print sheets, the interval between adjacent print sheets is determined on the basis of the size information of print sheets stored in a paper feed tray. The size of print sheets stored in the paper feed tray is determined with reference to the output of a sensor (for example, potentiometer sensor, reflective sensor, a photo-interrupter sensor or the like), and thereby non-standard size print sheets may not be accurately determined. If a print sheet of a size which is different from that as determined is transported, the next print sheet is transported thereafter in accordance with the scheduling arranged on the basis of the misdetermination so that paper jam may occur. There are print systems which simply determine the size of print sheets on the basis of the size information which is input through a panel or the like. In such systems, the possibility of misdetermining the paper size is increased.
Japanese Patent Published Application No. Hei 11-65370 discloses an image forming apparatus in which the above shortcoming can be solved. In the case of this image forming apparatus, the image forming operation is halted to avoid printing failure when operation mistake relating to the setting of print sheets in a paper feed cassette is detected by determining disagreement between the paper size which is designated by the user and the paper size which is actually transported from a paper feed tray.
However, in the case of the printing apparatus as described in Japanese Patent Published Application No. Hei 11-65370, when there is a disagreement between the result of paper size determination and the paper size as designated by the user, the image forming operation is always halted even if the print process can be continued by adjusting the image forming operation, the subsequent paper feeding timing and so forth. Accordingly, there is a problem that, even when only a minimum operational error occurs, the normal operation state has to be recovered by performing a predetermined recovering procedure which is troublesome to lower the productivity.
On the other hand, in recent years, a printing apparatus capable of improving the productivity during double-side printing has been developed. This printing apparatus is provided with a sheet reversing route and a scheduling function for improving the performance of double-side printing by inserting a print sheet, which has been printed on its main side and has been reversed upside down, between adjacent print sheets to be printed on their main sides for successively conducting the print process on main sides and the print process on back sides. In such a printing apparatus, when the above disagreement is detected, the productivity is substantially reduced.
Namely, in the printing apparatus provided with the sheet reversing route and capable of conducting the scheduling for improving the productivity during double-side printing, the paper feeding operations has to be performed at appropriate feeding intervals which are determined in accordance with the sizes of the respective print sheets to be printed in order to maintain the productivity. The feeding intervals are determined with reference to the size information of the print sheets stored in the paper feed tray, so that if a print sheet differing in size is accidentally transported, the print process is mistiming the back side printing operations. Since a print sheet which is reversed with the back side up is inserted between adjacent print sheets to be printed on their main sides in accordance with the scheduling, a number of print sheets to be controlled by the scheduling are adversely affected by misdetermination and paper jam which may occur due to the above described mistiming during the back side printing operations.